Equilibrium

Unit 5

What is equilibrium

Equilibrium is reached when rate of forward reaction = rate of reverse reaction, solution is balanced

Macroscopically: nothing happens

Microscopically: reactants are converted to products and products are converted to reactants

Why is equilibrium said to be dynamic

Because of the continual conversion of reactant to product

Exothermic reactants are hard to reverse if…

the change in heat is very large. It means that the Eareverse is also large.

Equilibrium must establish in what

A closed system

Why do some reactants have a %yield of less than %100

because reverse reactions can take place after products are formed

Example of Dynamic Equilibrium: Radioactive NaI\* in saturated NaI solution

Size of lump does not change (macroscopically) but overtime, solution becomes radioactive (microscopically). This indicates that NaI\* is dissociating and ions are moving out into solution. The ions from the NaI solution are crystalizing on the surface of the lump. So, the rate of dissociation (forward) and crystallization (reverse) are equal and it is at equilibrium.

Le Chatlilier’s Prinicple

If you disturb an equilibrium, it will shift to counteract the disturbance

Why will the disturbed equilibrium never reach its initial conditions?

because an equilibrium can only counteract up to %10. a new equilibrium will eventually establish

How to optimize the amount of product at equilibrium

flood reaction vessel while continuously removing product

Increasing the concentration of reactant or product

shifts equilibrium away from increase

Decreasing the concentration of reactant or product

shifts equilibrium towards decrease

As volume is … pressure is …

decreased, increased (inversely proportional)

An increase in pressure favours which direction

the direction with fewer moles of gas

what effect does pressure have in a reaction with the same amount of reactant and product moles

pressure has no effect

N2O4 (g)

the formation of N2O4 will be favoured since it has less moles. the reverse reaction is favoured

What is the equilibrium constant dependent on?

temperature

Endothermic reaction. Adding heat and removing heat

ΔH>0, heat is a reactant. Adding heat favours the forward reaction and cooling favours the reverse reaction

Exothermic reaction. Adding heat and removing heat

ΔH

why does a catalyst not affect the equilibrium

because it decreases the Ea forward and the Ea reverse the same amount

Equilibrium constant (k)

kc = \[products\]/\[reactants\]

kc

based on the molarities of reactants and products at equilibrium

unitless

large k vs small k

large k: more products present at equilibrium

small k: less products present at equilibrium

if k > 1

products dominate at equilibrium and equilibrium lies to the right

if k < 1

reactants dominate at equilibrium and equilibrium lies to the left

k of reaction in opposite direction

the reciprocal of the k in the other direction

Homogeneous Equilibria

when all reactants and products are in one state

Heterogeneous Equilibria

when one or more reactants or products are in a different state

Why do we ignore the concentrations of pure liquids, solvents and pure solids in equilibrium constant expressions

density and molar mass are not variables and we can’t find the concentration of something that isn’t a solution

To predict the direction of reaction

compare Q and K

Reaction not at equilibrium (Q)

Q = \[products\] / \[reactants\]

molarities at any time

if Q > k

reverse reaction occurs to reach equilibrium

shifts to left

if Q < k

forward reaction occurs to reach equilibrium

shifts to right

for Q to be large…

must have lots of product

for Q to be small…

must have lots of reactants

if Q = k

system is at equilibrium

The hundreds rule

initial concentrations / kc

if value is greater than 100, can ignore any x value added or subtracted